It might be DIY if you have the needed knowledge, skills and tools. The brick you want to cover that surrounds the firebox is noncombustible and part of the "clearance," the area around the firebox that must be noncombustible for safety reasons. You therefore will need to make sure that the mosaic you select and the gook you use to make it adhere to the brick are noncombustible and can withstand the heat emanating from the firebox.

Similarly, the "more substantial" mantel top you want to install should be about the same size as the one that is there now, because, if it is a much deeper shelf, it might not meet the "clearance" requirements. That of course depends on how high up from the upper edge of the firebox the mantel top is. The higher it is the deeper your shelf can be without any safety concern. With prefab fireplaces, the manufacturer usually gives the clearances in the owner's manual. But I suspect yours is not prefab, so you do not have that information. If you do want a deeper shelf, one way to find out if it will be safe is to place a thermometer just below the shelf when your fire is going full blast, and see how hot it gets. I do not know what a maximum safe temperature would be. If I were you, I would put your question in the forum in >. They have many knowledgeable people who are eager to be helpful.

Good luck.